A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; in mammals, these include:

The Latin term genitalia, sometimes anglicized as genitals and genital area, is used to describe the externally visible sex organs, known as primary genitalia or external genitalia: in males the penis, in females the clitoris and vulva.

The other, hidden sex organs are referred to as the secondary genitalia or internal genitalia. The most important of these are the gonads, a pair of sex organs, specifically the testes in the male or the ovaries in the female. Gonads are the true sex organs, generating reproductive gametes containing inheritable DNA. They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors.

A more ambiguously defined term is erogenous zone, subjectively, any portion of the body that when stimulated produces erotic sensation, but always prominently including the genitalia.

Thereafter, the development of the internal reproductive organs and the external genitalia is determined by hormones produced by certain fetal gonads (ovaries or testes) and the cells' response to them. The initial appearance of the fetal genitalia (a few weeks after conception) looks basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form the penile urethra.

In a larger perspective, the whole process of sexual differentiation also includes development of secondary sexual characteristics such as patterns of pubic and facial hair and female breasts that emerge at puberty. Furthermore, differences in brain structure arises, affecting, but not absolutely determining, behavior.